Conventionally, various interior decoration materials are laid on the sheet steel panels in the interiors of automobiles to improve the design quality and tactile impression.
While the function of the laid interior decoration materials is to cover the sheet steel panels in the interiors of automobiles to mainly improve the design quality as mentioned above, the interior decoration materials for an automobile are often required to function as a soundproof material which absorbs or blocks various noises generated as the automobile travels (such as road noise, engine noise, and wind noise). In particular, road noise and engine noise tend to intrude from the direction of the floor of the automobile. For this reason, various structures of laid interior decoration materials are proposed which are laid on the area from the floor panel to the peripheral walls to function to improve the sound absorption and sound insulation.
As a typical example, Published Japanese translation of PCT international publication No. 2000-516175 discloses a simple laminated body which is configured by laminating air-permeable materials and which has its sound absorption and sound insulation improved by controlling the total resistance of a particular layer among the laminated layers to flow of air. In this example, in order to provide a light weight sound insulating kit, heavy layers having a weight per unit area of 4.0 kg/m2 or more are eliminated from the constituent layers of the laminated body. A laminated body of an embodiment of the above document is formed by laminating air-permeable layers of weight per unit area of less than 2.67 kg/m2. In addition, Japanese Patent Application Laid-Open No. 2005-297703 discloses a technique of securing air-permeability of a laminated body by laminating layers made of air-permeable materials through a bonding layer in which apertures are formed.
However, the inventors of the present application conducted a test in real vehicles by laying interior decoration materials corresponding to the conventional example described in Published Japanese translation of PCT international publication No. 2000-516175 in the passenger chambers of a plurality of vehicles to find that there are times when quietness in the passenger chambers could not be optimumly secured. Particularly in cases where the vehicle was a diesel engine vehicle, often much engine noise intruded from the panels surrounding the passenger chamber (such as the floor panel) into the passenger chamber so that the passenger chamber could not be optimumly quiet.
Generally, sound absorption is evaluated according to the sound absorbing rate α=1−(Ir/Ii) wherein Ii is the intensity of sound incident to the material and Ir is the intensity of the reflected sound. In order to improve sound absorbing characteristics of a sound absorbing material, it is important to make voids in the material communicate with each other; whereby sound waves can be absorbed in the deep interior to attenuate the vibration energy of the sound. As a typical value which is a criterion for the degree of this communication of voids, the air-permeability rate (the reciprocal of the value of resistance to flow) is often used.
On the other hand, sound insulation is the characteristic to reduce the transmission of sound waves by blocking or reflecting incident sound, and is evaluated according to the transmission rate τ=(It/Ir) wherein Ii is the intensity of sound incident to the sound insulating material, Ir is the intensity of the reflected sound, and It is the intensity of the transmitted sound transmitted through the sound insulating material, and more practically is evaluated according to the transmission loss TL=10 log 10(1/τ). A heavy material having no voids is suitable for a sound insulating material in order to reduce transmitted sound, and therefore a material having the high density tissue is suitable for a sound insulating material.
In view of the above description, it is considered that in the case of an interior decoration material composed of only air-permeable materials, sound absorption can be secured but sound insulation is not sufficient since noise from outside the vehicle cannot be sufficiently blocked.
In addition, the interior decoration material of the conventional example described in Published Japanese translation of PCT international publication No. 2000-516175 has little formability. That is, it is difficult to form the interior decoration material of the conventional example into complex shape that adapts to the shapes at the locations where the material is laid in the interior of an automobile. In addition, the material has poor shape retention after being formed since it does not have a layer for retaining the formed shape.